i) Field of the Invention
The present invention relates to polarized light, optical and all related physical properties of a birefringent specimen, and in particular, to a polarized light method and device for determining the relative phase retardations, and the orientations of the optical axes of different layers in a multi-layered birefringent specimen, preferably, the relative phase retardation, which is related to the wall thickness, and the fibril angle of intact wood pulp fibres.
ii) Description of the Prior Art
A wood fibre, an example of a cellulosic fibre, is a biological material consisting of four principal layers: the primary wall P1, and the three secondary wall layers S1, S2 and S3 as shown in FIG. 1(a) [1]. All three secondary layers are composed of long crystalline cellulosic microfibrils, embedded in an amorphous matrix of hemicelluloses and lignin. The outer S1 and the inner S3 layers are very thin and their microfibrils are wound almost transversely to the fibre axis. The middle S2 layer, comprising 80-90% of the fibre-wall material, has cellulosic microfibrils wound in a helix at an angle, termed the fibril angle (θ), to the longitudinal fibre axis. The crystalline microfibrils are aligned in these layers, and are birefringent, making wood fibres birefringent. The magnitude of the birefringence depends on the thickness of the layers S1, S2 and S3, the orientations of their microfibrils, and the birefringence of each layer.
The fibre wall thickness and the fibril angle in the dominant S2 layer control the physical and mechanical properties of wood pulp fibres, and therefore strongly influence the response of pulps to papermaking treatments and the end-use properties of paper and board products. For instance, fibre wall thickness affects virtually all physical properties of paper including structural, strength and optical characteristics [2, 3]. Fibril angle, on the other hand, controls swelling/shrinkage properties [4], stress-strain behaviour [5] and dimensional stability of paper [6]. It has been shown that the S2 fibril angle strongly affects the collapsibility of fibres. The knowledge of important fibre properties such as fibre wall thickness and fibril angle is, therefore, critical for identifying and selecting resources that are optimal for a given end use. Unfortunately, due to the microscopic size of fibres, both fibre wall thickness and fibril angle are difficult to measure. Moreover, all fibre properties are heterogeneous in nature. The information on the distributions of fibre properties is considered to be very important in controlling pulp quality as it maps the extent of heterogeneity in a pulp, and allows identification of the amount of fibres with certain properties [2]. Thus, it is critically important to devise rapid techniques for quantifying individual fibre properties such as wall thickness and fibril angle in pulps.
Recently, a new instrument, the Kajaani FibreLab fibre analyzer, has provided measurements for fibre width and cell wall thickness of fibres flowing through a capillary tube [P1]. The principle of this instrument is based on the projected two-dimensional image of a fibre. This measurement technique is quite adequate for fibre width, which has dimensions in the range of tens of microns. However, this direct imaging technique faces many difficulties for accurate measurements of fibre wall thickness, which varies greatly from less than one μm to several μm. Recent investigation has shown that fibre wall thickness measurements from the Kajaani FibreLab are grossly incorrect [7].
The most reliable current techniques for determining wall thicknesses of wood pulp fibres are based on the fibre cross-sectional images, which can be generated by a Scanning Electron Microscope (SEM) on prepared fibre sections [8], or generated non-destructively by using the optical sectioning ability of confocal laser scanning microscopy (CLSM) [9]. When combined with image analysis, these techniques are capable of accurately measuring individual fibre transverse dimensions, such as wall thickness [4]. Although this technique provides valuable information on fibre transverse dimensions and is a good research tool, it is too slow for most practical industrial purposes. Thus, a rapid and accurate technique for measuring the wall thickness of individual wood pulp fibres is still lacking.
As mentioned earlier, fibril angle is another important fibre property. Several methods have been developed to measure fibril angle in wood pulp fibres: polarized-light microscopy [10], direct observation [11], micro-Raman spectroscopy [12], orientation of the elongated pit apertures [13], and most recently polarization confocal microscopy [13]. Although these techniques can provide measurements on fibril angles, they are also very slow.
Techniques based on polarized-light microscopy have been used for many years for measuring fibril angle in wood pulp fibres. These techniques make use of the natural birefringence of cellulose fibrils, and unequal retardations/refractive indexes in the directions parallel and perpendicular to the micofibrils. The direction of the fibrils in a single layer of the fibre wall can be readily obtained by examining the wall between crossed polars. However, this procedure requires a single wall; it cannot be used for intact fibres, as the opposite wall of the helically wound fibre interferes. This difficulty has been overcome, for example, by observing a single wall through a bordered pit, or by examining a single wall obtained by longitudinal microtome sectioning. A method for pulp fibres was developed by Page [10] in which a single wall is observed by reflecting light from mercury inserted into the fibre lumen. The fibril angle of the S2 layer is determined from the extinction positions for the (single) wall observed between crossed polars. Though simple in principle, this technique is tedious and hazardous, and is subject to errors from the S1 and S3 layers [14].
Most recently, developments based on transmission polarimetry techniques by Ye et al. [15, 16, 18] and Ye [P2, 17] claimed to be able to determine non-destructively the phase retardation Δ, and the fibril angle θ of the S2 layer on intact wood pulp fibres. There are many limitations on their methods. One major shortcoming of the above methods is that the influence of the S1 and S3 layers are neglected. In fact, the effects of the transversely wound S1 and S3 layers on the birefringence of intact softwood fibres are significant, particularly for thin-walled fibres, as shown by Page et al. [19]. It has also been shown by El-Hosseiny et al. [14] that the birefringent S1 and S3 layers, although thin, cannot be ignored in fibril angle measurements based on the polarized light method. Therefore, neglecting the effects of the birefringent S1 and S3 layers creates serious errors for measurements of both wall thickness and fibril angle. Moreover, as discussed in Ye's paper [17], the method based on a polarizer-sample-analyzer arrangement and the mathematical analysis by Ye at al. [15, 16] and Ye [P2] has many limitations. For example, the fibre sample in Ye's method [P2] has to be aligned to a certain orientation relative to the polarizer. Moreover, at least four intensity measurements at various analyzer orientations with the polarizer orientation fixed are required for calculating Δ and θ. Because the derived expressions for Δ and θ are in quadratic form, the results for Δ and θ are ambiguous. To avoid the ambiguity, the measurement has to be carried out for at least two wavelengths, and the user needs to distinguish the physically relevant results from two groups of intermediate ones. This method is not reliable, and can lead to misinterpretation of the data. Because of all these limitations, it will not work in an automatic, and definitely not in an on-line system. A new improved technique based on Muller matrix polarimetry was proposed by Ye [17]. Ye claimed that the newer method permits quantitative and non-destructive determinations of Δ and θ from measurements at one wavelength, and one advantage of his newer method is the feasibility of simultaneous measurements of several fibres at different orientations as the fibre orientation can also be obtained from the measurements. However, the method still needs many measurements obtained with the analyzer, polarizer and/or the retarders oriented at different angles, and it takes a very long time to make measurements on stationary fibres. Both of these techniques are very time consuming, and unsuitable for on-line type instruments.
The use of polarizing filters to generate a visual contrast for imaging birefringent fibres is not new. Many commercial fibre length analyser such as the Kajaani fibre length analyser (Kajaani Electronics Ltd, Finland), and the Fibre Quality Analyser (OpTest, Canada) [P3] have adopted such optical techniques for wood fibre length or/and shape measurements; individual fibres are imaged while they are flowing through a capillary tube or a flow-through cell. Although these instruments can measure fibre length rapidly, they cannot provide measurements on either fibre wall thickness or fibril angle. Therefore, there is still a need to develop a rapid and accurate technique for measuring fibre wall thickness and fibril angle of individual fibres in a way that is similar to the fibre length measurements.
The present invention aims at developing a new, rapid technique for measuring fibre wall thickness and fibril angle using a non-destructive optical technique that is based on circularly polarized light microscopy. The new invention provides a means to determine distributions of fibre properties because it is based on single fibre measurements. Properties of fibres are determined by analyzing the intensities of multi-wavelength light emerging from the system. This new invention can be automated, and implemented in a fibre flow-through system, thus allowing a rapid assessment of wood fibre properties (on-line in real time).